The present invention relates generally to the field of telephone line circuits and more specifically to a power drive circuit that is transformer isolated from the circuit which derives the drive signals, while not losing the dc reference. The present invention also relates to a transformer coupled drive circuit capable of driving power enhancement mode type FET devices and is advantageously applicable to ringing generators, particularly to multifrequency ringing generators, and to power supplies.
2. Description of the Prior Art
It is well known that low frequency signals can be amplified by factors of one-hundred or more by using pulse-width modulation techniques. For example, a low frequency signal may be pulse-width modulated by a high frequency triangular waveform. Examples of this technique may be found in U.S. Pat. No. 4,349,703, entitled "Programmable Ring Signal Generator", issued Sept. 14, 1982 and in U.S. Pat. No. 4,656,659, also entitled "Programmable Ring Signal Generator", issued Apr. 7, 1987. The pulses are usually at digital logic level values and are amplified by a power amplifier. The power amplifier includes switching circuitry to convert the logic level pulses to higher voltage pulses, typically at high voltage levels of about 350 volts. The high voltage pulses are then low-pass filtered and the original low-frequency signal is retrieved. An example of a programmable signal generator for generating an electrically isolated output voltage for use in a telephone subscriber line circuit may be found in U.S. Pat. No. 4,161,633, entitled "Subscriber Line/Trunk Circuit", issued Jul. 17, 1989; wherein a signal processor compares an output signal to a reference signal for the purpose of deriving a pulse duration modulated feedback control signal.
At present, power bipolar junction transistors and pulse transformers are used in the power amplifier stage wherein the low voltage pulses are converted to high voltage pulses. The transformers provide isolation which reduces the amount of switching noise feeding back to the drive circuitry, and provides a low impedance for driving the bipolar junction transistors. This technique has the disadvantage that the transistors themselves require a large amount of base current for operation, thereby causing the idle power of the circuit to be high, with consequent inefficiency of operation.
Another known technique is to utilize power, enhancement mode type Field Effect Transistors (FETs). These FETs, which function as switches, are faster and easier to drive. FETs usually require a gate-to-source voltage of 3-volts or more for operation, and they draw small amounts of gate current. A negative voltage between gate to source is usually required for turning the FET switch OFF. Present techniques utilizing such FETs do not incorporate a pulse transformer between the FET and the driving pulse-width modulation circuit. Therefore, such known circuits usually suffer from noise and component degradation. Another example of a circuit for generating a ringing signal which includes a pulse-width modulator to convert power amplitude signals to pulses of proportionate width is U.S. Pat. No. 4,399,499, entitled "Bi-Lateral Four Quadrant Power Converter", issued Aug. 16, 1983.